Mitchell Carson worked as Head of Defense at S.H.I.E.L.D., but was secretly an undercover HYDRA operative. In 1989, Carson along with Peggy Carter and Howard Stark met with Hank Pym about his Pym Particles at the under construction Triskelion. During their meeting, Carson made a snide remark about Pym's wife, Janet van Dyne, and got socked in the face by Pym.
In the present day, Carson was still after the Pym Particles, aligning himself with Darren Cross who has created a version of the Pym Particles. When Scott Lang as Ant-Man showed up and created a distraction, Carson managed to escape with Cross' Particles.
After he delivered the Pym Particles to HYDRA's last remaining heads following the betrayal of Helmut Zemo and Alveus Hive to follow their own agendas, he was then tasked to be one of the businessmen assigned to help Norman Osborn build the new organization HAMMER as a cover for their operations.